The effect of fertility control in the population dynamics and behavior of brushtail possums (Trichosurus vulpecula) in New Zealand

A large-scale field experiment began in late 1995 to determine the effectiveness of fertility control for managing brush tail possum populations in New Zealand. Six study populations received one of three sterility treatments, either 0%, 50%, or 80% of female possums sterilized by tubal ligation, with the level of sterility in each treatment maintained each year. Mark-recapture methods and analysis are being used to investigate the effect of the sterility treatments on demographic parameters such as population growth rate, survival, and recruitment. Recruitment of locally born young was successfully suppressed by sterility treatments, with lower yearling recruitment occurring on 80% and 50% sterility sites than on control sites. Negative population growth rate has been observed on one replicate set of 50% and 80% sterility sites, but growth rates have remained stable on the other replicate set of treatment sites. Examination of the contribution of immigrant animals to population growth rate has suggested that the settlement pattern of immigrants was compensating for reduced in situ recruitment. Analysis of survival rates revealed that sterile female possums had significantly higher survival rates than intact females, indicating a survival cost for reproduction. The effects of fertility control on the transmission rate of Leptospira interrogans serovar balcanica using a failure rate model revealed that the rate at which possums encounter Leptospira infection in the treatment sites (80% females sterile) was greater than the rate in control sites. As Leptospira infection is believed to be spread by direct contact, increased transmission in the 80% sterility sites suggests that contact rates between possums (mating contacts) are higher in areas subject to fertility control. Increased transmission is hypothesised to be driven by the increased frequency of oestrus resulting from the method of sterilization (tubal ligation) that was used to model the potential effects of immunocontraception. This result has direct relevance to both the epidemiology of disease as well as the potential spread of any biocontrol vector that relies on direct contact for transmission